As the information technology advances, to meet the growing demands on the telecommunication, the wavelength division multiplexing (WDM) technology is used to simultaneously transmit optical signals of various wavelengths on the existing transmission lines to expand the transmission capacity in addition to deploying new optical fiber cables. In WDM systems, the variable optical attenuator (VOA), integrated with micro-electro-mechanical-system (MEMS) chips and optical fiber, is commonly used. Most known MEMS VOA technologies are based on the tilt mirror to achieve optical attenuation; That is, an incident light coming from an incident optical fiber is reflected by a mirror to another outgoing optical fiber. When the mirror is tilted, part of the light is unable to be coupled into the outgoing optical fiber, leading to optical attenuation. Although the mirror-based MEMS VOA is widely common in industry, the technology is far from perfection. In actual application, there exists PDL and WDL problems, which cause the application limitation.
Light is an electromagnetic wave, and propagates energy through the electrical field oscillation. The oscillation direction of the electrical field is known as polarization. The light can be further categorized as a linearly polarized light (which has a single oscillation direction), a circularly polarized light or a elliptically polarized light (both have oscillation direction changes with time). Regardless of the polarization, a light can be decomposed into two orthogonal principal axes with relative phase difference, generally defined as P light (polarization parallel to the incident plane) and S light (polarization perpendicular to the incident plane).
Light of different polarization states may show different characteristics of transmittance, reflectance, refraction and diffraction when passing a medium. Hence, any optical element may cause PDL. The amount of PDL is defined, in general, as the subtraction of the minimum from the maximum of the insertion loss of all polarization states passing an optical element. The generation of PDL is mainly as a result of different energy loss caused by different transmittance, reflectance and diffraction of all polarization states on the propagation path. Furthermore, when lights passing anisotropic material having birefringent, different polarization lights will experience different propagation paths and lead to generation of PDL.
Moreover, as the attenuation increasing, light of different wavelengths will show different attenuation, that is, the attenuation is related to the wavelength. Therefore, WDL is defined as the attenuation difference between different wavelengths at a specific attenuation. For example, if the attenuation is 20.6 dB at the wavelength of 1525 nm and the attenuation is 19.4 nmdB at the wavelength of 1575 nm, the WDL near 20 dB will be 1.2 dB (=20.6-19.4). This is originated from the mode field diameter (MFD) of a longer wavelength light is greater than the MFD of a shorter wavelength in the single mode fiber (SMF) common used nowadays. As a result, the attenuation of longer wavelength is less than the attenuation of the shorter wavelength at a fixed beam offset causing attenuation. In actual application, the WDL issue restricts the system application.
FIG. 1 shows a schematic view of the structure of a conventional rotating mirror variable optical attenuator, including a pigtail 10, with a first waveguide for incident light and a second waveguide of receiving returned light; a lens 11, fixed to the front end of the pigtail to focus the incident light from the first waveguide of the pigtail 10 and to make the reflected light return to the second waveguide of the pigtail 10; a cap 12, with a glass window 13 at the center, coated with an anti-reflection film, or a slant surface to prevent the reflected light from coupling to the optical fiber; a tube 16, to fix the lens 11 and the cap 12; and a chip 15 with a rotating mirror, fixed to a header 14 to reflect the incident light passing the lens from the first waveguide, and make the reflected light pass through the lens to return to the second waveguide. For general process, the lens and the pigtail are assembled as a collimator, and then assembled to the tube and the cap. Because the stress between the tube and the lens interface will cause light of different polarizations to experience different reflection indices to form different paths, the PDL is resulted.
FIG. 2 shows a schematic view of another structure of a conventional rotating mirror variable optical attenuator, including a pigtail 20, having a first waveguide for incident light and a second waveguide for receiving returned light; a lens 21, fixed inside a cap 22 to focus the incident light from the first waveguide of the pigtail 20 and to make the reflected light return to the second waveguide of the pigtail 20; and a chip 24 with a rotating mirror, fixed to a header 23 to reflect the incident light passing the lens from the first waveguide, and make the reflected light pass through the lens to return to the second waveguide. Because the stress between the cap 22 and the lens 21 interface will cause light of different polarizations to experience different reflection indices to form different paths, the PDL is resulted.
Other patents disclosed PDL compensation methods. For example, U.S. Pat. No. 6,266,474 disclosed a variable optical attenuator using a wedge type neutral density filter. In this type of VOA, because the light enters the wedge type neutral density filter with an angle greater than 10°, the wedge filter becomes a polarization related element. To compensate the PDL caused by the wedge filter, a bi-planar lens having two parallel surfaces is disposed on the optical path. The tilt direction of the bi-planar lens is opposite to the tilt direction of the wedge filter to compensate the PDL caused by the wedge filter. However, this type of compensation is only applicable to compensate a steady PDL source, and not suitable for compensating non-fixed PDL caused by assembly stress.
In addition, for WDL compensation, U.S. Pat. Nos. 7,574,096, 8,280,218 and Publication No. US20040008967 all disclosed a structure using material dispersion feature to make the offset of the short wavelength optical spot less than that of the long wavelength at a fixed attenuation. Because the WDL compensation designs disclosed in the prior arts may result in the WDL compensation inconsistent with desirable due to the material tolerance and assembly variation, it is imperative to provide a VOA to reduce PDL and WDL.
The present invention provides a WDL compensation method to compensate WDL in real-time during assembly to overcome the material tolerance and assembly variation. That is, WDL can be minimized by adjusting the tilt angle θt of the pigtail.